High speed web type printing presses consist of two side frames which are bolted together by spacer pieces. The frames carry several high peripheral speed cylindrical rollers, some of which hold a master print form. These master print forms are coated with various colors of ink. The inks are typically solvent based and are mildly corrosive to the face of the rollers. Most of the rollers used are non stainless steel. Their diameters are from 4 to 16 inches and they may be up to 96 inches in length.
Each roller contains a cut deep channel or slot which is parallel to the longitudinal axis of the roller. Into this slot is inserted a "locking" device which positions and firmly fastens the master print form to the high speed turning roller. The outer surface of the roller is coated with a hard chrome surface which prevents chemical corrosion of the cylinder body. In addition the opposed walls of the slot are also chromed a fixed distance from the outer surface of the roller inwardly. It is extremely difficult to plate the walls of the slot and particularly the edges defined by the roller surface and the walls of the slot. It has been found that these edges and wall surfaces typically have pin holes and/or blisters. The solvents in the inks permeate through the pin holes and/or blisters and corrode the underlying surface. The corrosion results in the discoloration or streaking of the master print forms. When this corrosion condition exists, the cylinders must be replaced or reconditioned.
One solution would be the use of solid stainless forgings for the rollers but the cost is prohibitive. Another possible solution is stainless coatings of the surface of the cylinder and a stainless overlay of the slot opening. However, this practice is "blocked" metallurgically because the two interfaces are not metallurgically bound together such that they have a contiguous grain structure. Internal stresses at the grain boundaries of the respective welds develop allowing the solvents to permeate through, corroding the underlying surface.
The invention described herein is a method for coating cylindrical rollers that carry master print forms and the rollers produced by the process. A welded interface between a stainless covered roller surface and a weld overlay of the slot opening results in a homogeneous mixture of stainless welding free from internal stresses and produces a continuous uniform grain structure. The invention provides both a surface free of pin holes and bubbles and eliminates the need for the chrome plating of rollers.
The invention broadly comprises forming a weld preparation groove in the surface of a cylindrical roller, said groove being parallel to the longtitudinal axis of the roller. The groove is filled with a weld material. Subsequently, the outer surface of the cylinder is welded with a compatible welding material to coat the cylinder surface. The weld materials both for the groove and the surface are compatible such that when the surface is welded, the weld material in the groove is recast to form a homogeneous grain structure with the surface weld. The slot is then formed and the surfaces of the facing walls of the slot, the slot edges and the roller surface are uniformly coated.
In a preferred embodiment, the weld material is stainless steel. However, the weld material may be selected from the following materials, which may be used alone or in combination, belonging to the stainless steel family and is referred to as the austenitic and martensitic chromium nickel group. As long as the weld material in the groove has the characteristic of being able to be recast when the roller surface is coated by welding and to reform into a grain structure which is homogeneous with the grain structure of the material used on the surface then that material is within the scope of the invention.